This invention generally relates to a nickel/hydride battery with positive electrodes of nickel-hydroxide and with negative electrodes of a hydrogen storing alloy, stacked in a gas-tight housing with alternating polarities.
Similar to gas-tight nickel/cadmium batteries, the gas-tight operation of a nickel/hydride battery presumes the continuous removal of oxygen formed during the transfer of charge to the positive electrode. The amount of oxygen formed depends heavily on the conditions of charging and discharging, the temperature, the composition of the electrolyte and cell balance. In equilibrium, the charging current is fully consumed at a set oxygen pressure. This oxygen pressure is set under certain operating conditions and for a certain equalization rate for the electrodes.
Since the negative electrode is over-sized relative to the positive electrode, even after being fully charged, the positive electrode will still contain a negative charge reserve. Likewise, upon complete discharge of the cell, the negative electrode will remain partially charged, as a negative discharge reserve. This assures that in the event of overcharging or over-discharging, discharging, oxygen or hydrogen, respectively, will be evolved only on the positive nickel electrode. Such gases must then be decomposed on the surface of the negative electrode, at that electrode's potential.
By way of example, such decomposition may be accomplished electrochemically, according to the equation EQU 1/2O.sub.2 +H.sub.2 O+2e.sup.- .fwdarw.2OH.sup.-
or by chemical combination of the oxygen with stored hydrogen (H.sub.st), according to the equation EQU 4H.sub.st +O.sub.2 .fwdarw.2H.sub.2 O.
In either case, it is important that the removal of such gas (especially of the oxygen) take place as rapidly as possible so that no impermissibly high gas pressure, and no oxy-hydrogen gas can form in the interior of the cell.
German Patent 1,671,972 discloses a gas-tight alkaline battery in which the area of the negative electrode is enlarged by dividing the electrode in two, so that the resulting (partial) electrodes lie side by side. A frame-like spacer is positioned between the partial electrodes, developing a gas consumption space between the partial electrodes. Passageways are provided for receiving gases for delivery to the gas-consumption space so that oxygen gas evolving from the neighboring positive electrodes can rapidly react on the walls of the partial electrodes.
German Patent 2,838,857 discloses a metal oxide/lanthanum nickel hydride storage cell in which oxygen is reduced by an auxiliary electrode which is in electron contact with the negative electrode, in each case inside the electrode stack but spatially separated from it. The disclosed electrode arrangement has two different types of separators. Coarse porous hydrophobic separators are in each case located between positive electrodes and auxiliary electrodes, and assure the access of oxygen to the auxiliary electrodes. Hydrophilic separators with low gas permeability are located between the positive and negative electrodes, to protect the negative electrodes against direct contact with oxygen.